Sustainability assessment has become a crucial component of public policy, urban planning, and research on sustainable development. Most assessment methodologies remain predominantly outcome-oriented, evaluating the state of sustainable development while paying relatively little attention to the governance processes through which it is pursued. This paper argues that sustainable development may be understood and evaluated as a continuous governance process rather than solely as a set of societal outcomes. This study proposes the Governance-Centered Sustainable Development Assessment as a theoretical framework for evaluating sustainable development through governance rather than solely through outcomes. The framework posits four governance domains: governance capacity, governance implementation, governance effectiveness, and sustainable development outcomes within a unified governance architecture. A standardized methodology operationalizes this perspective integrating longitudinal archival data, governance activity coding, structured evaluation worksheets, evidence hierarchies, process tracing, and a municipal sustainable development governance maturity scale. The methodology is demonstrated through a longitudinal application to a city. This research advances an ontological reconceptualization of sustainable development as a governance process; and it extends program evaluation theories to sustainability governance by providing a structured, transparent and transferable framework for evaluating municipal governance processes alongside community outcomes. These contributions shift sustainability assessment from measuring how sustainable communities are to understanding how communities govern sustainable development.